This invention pertains to aluminum aerospace alloys. More particularly, this invention pertains to aluminum alloys that are suitable for welding, yet have improved performance properties, particularly corrosion resistance.
Airplane manufacturers are investigating the possibility of welding fuselage skin panels together as a low cost alternative to fastening them with rivets, welding generally being defined as having good retention of mechanical properties after the joining together of two or more parts, either by mechanical welding, laser welding, other welding techniques, or a combination of practices. Existing alloys that are currently used for fuselage skins include Aluminum Alloys 2024 and 2524, Aluminum Association registrations. Certain properties of these alloys are adversely affected by welding, however. Alloy 6013 has attractive mechanical properties for use as a fuselage skin alloy and is also weldable. But alloy 6013 is susceptible to intergranular corrosion attack which can increase local stress concentrations when the aircraft into which 6013 is installed gets subjected to stress conditions such as repeated pressurization/depressurization of a plane""s fuselage flight after flight. Cyclic, or repetitive, loading can lead to the formation of fatigue cracks at these sites in less time than would be expected for an uncorroded structure. In order to take full advantage of the cost savings offered by fuselage skin panel welding, therefore, it would be desirable to develop a weldable aluminum aerospace alloy that has improved resistance to intergranular corrosion attack.
Other patents or international applications are applicable to this alloy system and product application. Comparative alloy compositions are listed in Table 1 that follows.
A principal objective of the present invention is to provide an improved 6000 series alloy that is weldable, yet exhibits improved corrosion resistance properties. It is another principal objective to provide an improved aluminum aerospace alloy suitable for forming: into sheet and plate products primarily, into various extruded product forms secondarily, and less preferentially into forged product shapes using known or subsequently developed product manufacturing processes.
These and other objectives are met or exceeded by the present invention, one embodiment of which pertains to an aluminum alloy suitable for welding. That alloy consists essentially of: about 0.6-1.15 wt. % silicon, about 0.6-1.0 wt. % copper, about 0.8-1.2 wt. % magnesium, about 0.55-0.86 wt. % zinc, less than about 0.1 wt. % manganese, about 0.2-0.3 wt. % chromium, up to about 0.2 wt. % iron, up to about 0.1 wt. % zirconium and up to about 0.1 wt. % silver, the balance aluminum, incidental elements and impurities. On a more preferred basis, this alloy contains 0.7-1.03 wt. % silicon, about 0.7-0.9 wt. % copper, about 0.85-1.05 wt. % magnesium, about 0.6-0.8 wt. % zinc, about 0.04 wt. % or less manganese, about 0.21-0.29 wt. % chromium, about 0.15 wt. % or less iron, about 0.04 wt. % or less zirconium and about 0.04 wt. % or less silver, the balance aluminum, incidental elements and impurities. Originally, it was believed that silicon minimums of about 0.75 wt. % would suffice. Subsequent samplings have revealed, however, that silicon levels as low as 0.6 wt. % should also work in conjunction with this invention. it is believed that the addition of chromium and significant reduction of manganese in this composition are pertinent to the results achieved.
The invention consists of an aluminum alloy having a composition as listed in the above table. This alloy offers increased typical tensile strength compared to existing alloys when aged to a peak temper or T6 condition. For comparative purposes, the relative T6 typical strengths and % elongations for various alloys are listed in Table 2 below. Minimum or guaranteed strength values cannot be compared versus 6013 values as not enough statistical values exist for fairly determining such minimum or guaranteed strength values for the invention alloy herein.
In the peak aged condition, the alloy of this invention offers greater resistance to intergranular corrosion resistance compared to its 6013 aluminum alloy counterpart. Further increases in intergranular corrosion resistance can be obtained by underaging, i.e. purposefully limiting artificial aging times and temperatures so that the metal alloy product does not reach peak strength.